There are many industrial applications using conductive metal regions on substrates, many of which require the conductive metal regions to be formed according to a pattern. An important application is the manufacture of printed circuit boards, where metal layers are formed into a pattern on a substrate to connect electrically different components and electrical devices according to a predetermined arrangement. Other applications include aerials and antennae, such as those found in mobile telephones, radio frequency identification devices (RFIDs), smart cards, contacts for batteries and power supplies, arrays of contacts for flat screen technologies (liquid crystal displays, light emitting polymer displays and the like), electrodes for biological and electrochemical sensors, smart textiles, conductors used in displays, heater elements, interconnects and decorative features. In many of these applications it is important to be able to control closely the size and form of the conductive metal regions. In most of these applications, the metal region must be conductive and a high level of conductivity is desirable, or in some cases essential. In addition close control over the physical properties of the metal regions, e.g. accurate reproducibility of resistance, may be important.
It is known to form conductive metal regions on substrates by the reduction of metal ions. This is the basis of the so-called “electroless” plating or deposition procedure in which an activator such as a catalyst or catalyst precursor is applied to a substrate which is then immersed in a succession of baths. One of the baths comprises a metal ion (e.g. a copper salt), a reducing agent (e.g. formaldehyde) and a base to activate the formaldehyde (e.g. sodium hydroxide). The metal ion is reduced to form a conductive metal region on the substrate surface, where the activator has been applied.
WO 2004/068389 (Conductive Inkjet Technology Limited) proposes an alternative to immersion procedures, in which metal ion and reducing agent are deposited together on a substrate, preferably by inkjet printing, and react in situ, in the presence of an activator, to form a conductive metal region.
WO 2005/045095 and WO 2005/056875 (both of Conductive Inkjet Technology Limited) disclose piezoelectric inkjet printing on a substrate of non-aqueous mixtures of ultra-violet (UV)-curable monomers and oligomers together with palladium acetate activator (catalyst precursor) to form an activator-containing region on the substrate, and subsequent reaction, e.g. by electroless deposition, to produce conductive copper regions on the substrate.
Use of inkjet printing for deposition of activator enables close control over the size and form of activator on a substrate, and hence of the size and form of the resulting metal regions.
WO 2005/010108 (Hewlett-Packard) discloses inkjet printing of non-curable compositions including palladium aliphatic complexes for catalysing electroless deposition of copper.
Inkjet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive inks to produce a stream of drops of electrically charged ink that are deflected by an electric field to an appropriate location on a substrate. In DOD processes, individual drops of ink are expelled from the nozzle of a print head either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a bubble (in thermal inkjet printing, also known as bubble jet printing). Thermal inkjet printing has advantages over piezoelectric inkjet printing, with printers and print heads being lower cost and with the printing process being able to achieve better resolution.
Inkjet inks need to satisfy a number of requirements, including the following:                Viscosity must be appropriate. With DOD inks there are greater limitations on inks for thermal printing than for piezoelectric printing, with it generally being necessary for inks to have a viscosity of below about 4 mPas at print head operating temperature (which is typically 40-50° C.), which usually equates to a viscosity of less than 6.5 mPa.s at room temperature (25° C.), to be capable of being thermally inkjet printed. In this specification, all viscosity values are at 25° C. unless otherwise specified.        The ink must not cause unacceptable levels of clogging or blockage of printing nozzles.        The ink must not result in build up of deposits on the ejection heaters of thermal inkjet print heads (a process known as “kogation”) to an unacceptable level during the working life of a print head.        The ink should be stable in storage, without settling out or coagulation of materials.        
Curable DOD inks are known. These typically comprise one or more monomers etc. curable in response to appropriate conditions, typically ultra violet (UV), infra red (IR) or heat.
U.S. Pat. No. 5,623,001 (Scitex) discloses UV-curable DOD inkjet inks, particularly for piezoelectic printing, comprising water (20-75%) and water-miscible UV-curable monomer and/or oligomer e.g. acrylic materials (20-60%). The document makes no reference to thermal inkjet printing, and does not teach how to produce inks with a viscosity of less than 6.5 mPas, suitable for thermal inkjet printing, nor does it teach how to prevent kogation.
U.S. Pat. No. 5,952,401 (Canon) discloses curable water-based inks for piezoelectric and bubble jet printing using curable monomers. The document does not address the issue of prevention of kogation caused by curable materials.
U.S. Pat. No. 6,294,592 (BASF) discloses curable inkjet inks exemplified by UV-curable polyurethane dispersions. The document does not address issues of viscosity or prevention of kogation
None of the above three documents discloses use of inkjet inks as a vehicle for carrying an activator.
Curable monomers and oligomers tend to have limited solubility/miscibility in water, and substantial practical difficulties arise in producing water-based inks with sufficiently low viscosity (below 6.5 mPa·s) to be useful for thermal inkjet printing that do not undergo undesirable phase separation, do not cause clogging or blockage of printing nozzles, and that do not result in unacceptable levels of kogation.
We have found that by use of appropriate curable material(s) together with water-compatible solvent (referred to as a co-solvent) for the curable material(s) it is possible to produce low viscosity compositions suitable for thermal inkjet printing. These compositions can be used as a vehicle for carrying activator, enabling inkjet printing of an activator on a substrate for subsequent reaction to form a conductive metal region, the size and form of which can be readily regulated.